Preconditioning FEM discretisations of the high-frequency Maxwell equations by either perturbing the coefficients or adding absorption

Abstract: We prove bounds on $\mathsf{I} - \mathsf{A}2^{-1}\mathsf{A}_1$ where $\mathsf{A}\ell$, $\ell=1,2$, are the Galerkin matrices corresponding to finite-element discretisations of the time-harmonic Maxwell equations $k^{-2}{\rm curl} (\mu_\ell^{-1}{\rm curl} E_\ell) - \epsilon_\ell E_\ell =f$; i.e., we consider the situation where the Maxwell FEM matrix is preconditioned by the FEM matrix arising from the same Maxwell problem but with different coefficients. An important special case is when the perturbation consists of adding absorption (in the spirit of "shifted Laplacian preconditioning" for the Helmholtz equation). The results of this paper are the Maxwell analogues of the Helmholtz results in [Gander, Graham, Spence, 2015] and [Graham, Pembery, Spence, 2021], and confirm a conjecture in the recent preprint [Li, Hu, arXiv 2501.18305]. These results are obtained by putting the Maxwell problem in an abstract framework that also includes the Helmholtz problem; as a byproduct we weaken the assumptions required to obtain the Helmholtz results in [Gander, Graham, Spence, 2015] and [Graham, Pembery, Spence, 2021].